Nano ceramics are ceramics which have a mean grain size in the range 1-200 nm. Advantages of nano ceramics are known and include that they exhibit greatly improved mechanical and optical properties along with other phenomena that traditional sub-micron ceramics do not exhibit. Work performed by Binner et al (US 2011/0230340 A1) asserts that yttrium doped zirconia ceramics with a mean grain size of 190 nm or less exhibit hydrothermal resistance along with excellent wear resistance properties. These enhanced properties make nano ceramics excellent materials for a wide range of applications including use in the electronics industry in phone cases, glasses and watch cases, in the abrasives market through tougher wearing materials and in the healthcare market such as in implants. One application of nano ceramics comprising zirconia is as a dental ceramic material. For example, US 2016/0095798 A1 and U.S. Pat. No. 9,820,917 B1 relate to zirconia dental ceramics which are formed from suspensions of zirconia nanoparticles, and gels made from such suspensions. The ceramics described in these documents are stated as having translucency, opalescence and desirable physical properties. US 2016/0095798 A1 also contains an extensive discussion of earlier patents relating to zirconia dental articles, as well as patents, patent applications and journal articles in the more general field of zirconia ceramics and processing methods.
The process of forming a zirconia dental article which is described in US 2016/0095798 A1 involves the steps of (i) forming a zirconia nanoparticle suspension into a suitable shape to form a wet zirconia green body, (ii) drying the wet green body to produce a zirconia green body, (iii) heating the zirconia green body to for a zirconia brown body, and (iv) sintering the zirconia brown body to form an opalescent zirconia sintered body.
A problem with drying nanomaterials such as those described in US 2016/0095798 A1 and U.S. Pat. No. 9,820,917 B1 is that the particles can aggregate and no longer possess the properties of a true nanoparticle. To try and overcome this problem, US 2016/0095798 A1 mentions in Example 6 subjecting the suspensions to 15 days drying to slow down the process. This process is not only time consuming but expensive should parts fail during the lengthy drying times.
A potential problem with this extended drying process is that when a dense green body has been formed, de-binding must occur to remove any organics or volatiles from the ceramic. A dense green body will result in smaller voids in the material and if these are not sufficiently large enough to allow the organics or volatiles to escape, pressure can build up inside the body and lead to cracking and failures in the resulting ceramics.
It is desirable to minimise the number of failures in the resulting ceramics in order to reduce the cost of producing zirconia nano ceramics. Methods and materials for reducing the failure rate of zirconia dental ceramics have therefore been sought.